Template Assisted Synthesis Of Fe-N-C Catalysts For Oxygen Reduction Reaction And Electrochemical Mechanism Research

The process of oxygen reduction reaction?ORR?is quite distinct due to the difference in electrolyte and electrode materials.At present,most reported non-noble metal ORR catalyst materials can only exert excellent performance in alkaline electrolytes,but the researches under acidic and neutral conditions are rare.Nowadays,proton exchange membrane fuel cells?PEMFCs?that need to be used in acidic electrolytes have certain advantages in the field of new energy.However,the Pt-based precious metal catalysts used in PEMFCs have been seriously hindered in their commercialization due to scarce reserves,high prices,and easy poisoning and deactivation.Therefore,the research on replacing Pt-based precious metal catalysts has become a global hot research topic.In response to the above problems,the development of high-performance and long-life non-noble metal-based catalysts to replace the application of Pt/C in fuel cells is urgently needed.At present,Fe-N-C catalyst has a high selectivity for the four-electron reaction path of ORR due to its Fe-N₄ structure,and thus has become the most promising non-noble metal oxygen reduction catalyst.According to the design requirements of non-noble metal ORR catalysts,in order to develop low-cost catalysts with stability and high active specific surface area under wide pH conditions,we prepared a series of Fe-N-C catalysts by template-assisted method and he catalytic mechanism of these catalysts was further studied.The main research contents are as follows:?1?In this thesis,the Fe-N-C catalysts were prepared via in situ polymerization of aniline monomer on the surface of SiO₂ templates followed by the pyrolysis under N₂or NH₃ atmosphere.The monodispersed SiO₂ nanospheres were employed as the template and the polyaniline?PANI?was deem as carbon and nitrogen source.By optimizing the reaction conditions and adjusting the amount of oxidant and iron source,a series of N-C and Fe-N-C-x catalysts were successfully synthesized.The physicochemical properties of the materials were studied by SEM,TEM,XRD,Roman,BET and XPS technologies.The high ORR activity catalyst is the one with pyridine nitrogen content 29.9%.Combined with cyclic voltammetry and linear scanning voltammetry,the electrochemical performances of these materials were evaluated,and finally the Fe-N-C-10 catalyst with superior performance under acid and alkaline medium was selected.?2?Further optimization was conducted by introducing a double template method with a layer of SiO₂ template coated on the surface of SiO₂@PANI-Fe,which aims to suppress the volatilization of nitrogen-containing pieces in carbon organic compounds and prohibits the free migration of metal atoms at high temperatures.SEM and TEM results indicate that SiO₂@PANI-Fe@SiO₂ precursor?D-Fe-N-C-x?was successfully prepared with a core-shell structure.XPS results indicate that the surface of D-Fe-N-C-10 catalyst material has higher nitrogen content amount?6.6%?and more Fe-N_xstructure sites.Cyclic voltammetry and linear cyclic voltammetry results showed that D-Fe-N-C-x catalyst had good performance both in basic and acidic conditions.In addition,the methanol resistance and durability of D-Fe-N-C-10 catalyst under alkaline conditions is better than that of the Pt/C catalyst,so it is suitable for alkaline fuel cells and metal-air batteries.?3?Finally,on the basis of the works in the previous two chapters,the preparation process was further optimized.The precursor SiO₂@PANI-Fe-10 with better performance was used to prepare a single atom catalyst?Fe-N-C-NH₃?with the help of the NH₃ atmosphere provided by pyrolysis of urea at high temperature.HAADF-STEM and EESL successfully proved the existence and distribution of single atoms.The results of BET and BJH showed that the single-atom catalyst has a high specific surface area(436.7 m² g^-1)and a microporous structure??2 nm?.XPS fitting results showed that the electronic structure of the single-atom catalyst material surface is rich in pyridine N?40.7%?,graphite N?45.4%?,and Fe-N₄ active sites.The catalytic activity of the single-atom catalyst at full pH?acidic,neutral and basic?and methanol tolerance and durability tests were analyzed by RDE and RRDE electrochemical testing techniques,and compared with the commercial Pt/C catalyst.The results showed that the single-atom catalyst has superior activity and high selectivity for the four-electron reaction path towards ORR,which is comparable with the Pt/C catalyst at all pH.